Single-Impact Electrochemistry for Bacterial Sensing

Abstract

The main objective in the detection and analysis of bio-entities such as bacteria is a low detection limit, a short assay time and a high selectivity in identification, ideally combined with easy-handling and cheap instrumentation. In this way, the development of innovative analytical methods in terms of high sensitivity and spatial-temporal resolution allowing both qualitative and quantitative analysis at single-cell and subcellular levels is becoming a technological requirement. The main advantage of electrochemistry in biosensing applications is to combine high sensitivity and ease of use with the possibility of miniaturization at low cost. Especially, new electrochemical techniques and various types of electrodes have been recently developed offering the feasibility of analysis at the single-cell level with high sensitivity and selectivity. Nano-electrochemistry has been widely extended to various applications such as biosensing thanks to the continuous improvement of the instrumentation sensitivity, especially for individual entity electrochemical detection. Single-impact electrochemistry provides a low limit of detection (in principle, one single species) inherent to this electro-analytical method and the ability to study single entities (cells, viruses, bacteria, nanoparticles...) in real-time through a dynamic measurement [1]. The electrochemical nano-impacts method or discrete collisions technique (stochastic events) consists in detecting single impacts of various entities such as nanoparticles, cells, bacteria, vesicles, viruses, proteins in solution at a polarized ultramicroelectrode (UME) [2]. For each collision event, a specific signal is recorded in the chronoamperometry curve (current as a function of time) corresponding to an “impact” of the entity onto the UME surface. Crucial information can be reached from the analysis of the electrochemical impact event in the amperometry measurement, such as the concentration and the size of the colliding entity.Electrochemical nano-impacts (or collisions) of single bacteria are usually detected by recording a chronoamperometry measurement (i–t curve) at a UME biased at a redox potential of an electroactive species in an aqueous electrolyte containing between 106 and 109 bacteria per milliliter [1, 3]. Since 2015, most of the reports in this area concern Escherichia coli (E. coli) as a model bacterium but this sensitive electro-analytical technique has not proved to be efficient for selective detection yet, except for the discrimination between Gram-positive and Gram-negative bacteria. In order to develop single-impact electrochemistry toward the identification of bacterial cells, several studies deal with the use of redox mediators for probing the electrochemical activity of the bacterium. Also, an original and recent approach is to detect the virulence factors released by pathogenic bacteria rather than the cells themselves, based on redox liposomes single-impact electrochemistry [2, 4].